mm/memory_hotplug: fix updating the node span

BugLink: https://bugs.launchpad.net/bugs/1854216
commit 656d571193262a11c2daa4012e53e4d645bbce56 upstream.

We recently started updating the node span based on the zone span to
avoid touching uninitialized memmaps.

Currently, we will always detect the node span to start at 0, meaning a
node can easily span too many pages.  pgdat_is_empty() will still work
correctly if all zones span no pages.  We should skip over all zones
without spanned pages and properly handle the first detected zone that
spans pages.

Unfortunately, in contrast to the zone span (/proc/zoneinfo), the node
span cannot easily be inspected and tested.  The node span gives no real
guarantees when an architecture supports memory hotplug, meaning it can
easily contain holes or span pages of different nodes.

The node span is not really used after init on architectures that
support memory hotplug.

E.g., we use it in mm/memory_hotplug.c:try_offline_node() and in
mm/kmemleak.c:kmemleak_scan().  These users seem to be fine.

Link: http://lkml.kernel.org/r/20191027222714.5313-1-david@redhat.com
Fixes: 00d6c019b5bc ("mm/memory_hotplug: don't access uninitialized memmaps in shrink_pgdat_span()")
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index a2466fc1534e561a510f5fc24a2aaf28795c629e..e3b096dad3b7750cad5dffb2543e191b8c347e5c 100644 (file)
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -472,6 +472,14 @@ static void update_pgdat_span(struct pglist_data *pgdat)
                                             zone->spanned_pages;
 
                /* No need to lock the zones, they can't change. */
+               if (!zone->spanned_pages)
+                       continue;
+               if (!node_end_pfn) {
+                       node_start_pfn = zone->zone_start_pfn;
+                       node_end_pfn = zone_end_pfn;
+                       continue;
+               }
+
                if (zone_end_pfn > node_end_pfn)
                        node_end_pfn = zone_end_pfn;
                if (zone->zone_start_pfn < node_start_pfn)